In some electronic equipment it is necessary to have different feedback control characteristics for the gain control elements of two or more automatic gain control (AGC) circuits. One example of such equipment is an extended range automatic cable equalizer. Such equalizers are usually constructed with two gain control stages connected in tandem. This is necessary because the gain range of JFET (junction field-effect transistor) devices, which are typically used as variable resistance gain control elements in AGC circuits of the the gain stages, have inadequate range over which they are reasonably linear. Such tandem stage arrangements are described, for example, in U.S. Pat. No. 3,824,501 issued July 16, 1974 to C. A. Harris, in U.S. Pat. No. 3,652,952 issued Mar. 28, 1972 to W. Chen, and in pending application Ser. No. 658,773 by T. J. Aprille et al., now U.S. Pat. No. 4,606,043, all of which are assigned to the same assignee as is the present invention.
One problem with present equalizers is that the JFET variable resistance control elements of the AGC circuits are not readily integratable in a single chip with the other active circuit devices, since they require different processing steps during manufacture. Their provision as discrete off-chip devices significantly increases the size and cost of the equalizer and also degrades the precision of the equalization because of reduced correlation between gain-determining parameters.
Another problem with present equalizers is that, even if all the active devices were integrated, there would be a need for connection of a relatively large number of off-chip capacitors which are too large in value to be readily integrated on the circuit chip. The connection of these capacitors requires the circuit chip to be packaged with a larger number of connection pins, thereby also contributing to the size, complexity, and cost of the equalizer.